The only thing that locks them into a class are the commands. Otherwise, they all learn the same spells that one another have. Materia prevents this to a degree. If someone is dead, the next turn up is a revive spell, be it shadow or edgar.
And equipment. Let's look at FF7, Cloud can use Steal, Tifa can use Steal, Barret can use Steal... Anyone can be setup to be a thief. Such is not a case in FF6. Cloud can know 0 magic spells, or every spell in the game with Master Magic. Cloud doesn't know Life2 once Life materia is taken off of him. Locke still knows Life2 when Phoenix is taken from him. See the difference? It's huge. You can have Cloud with both nothing and everything should you work for it. Locke will always know Steal/Capture/Mug and Magic.
What is the point of bringing up Celes and her OP equipment into a discussion about the differences between materia and espers? To further differentiate characters who already all learn the same spells with no restrictions?
Because her equipment differentiates her from other people because of the system utilized in 6. Not everyone can wear everyone's stuff in FF7. Spells aren't the only skills that you could equip in FF7... I'm really getting the feeling you never played FF7 at this point.